The present invention relates to an electric pump unit formed by unitizing an electric motor and an internal gear pump, which is driven by the electric motor to draw and discharge fluid such as oil, and to an electric oil pump apparatus having the electric pump unit.
As means to deal with global environmental problems, electric oil pump apparatuses are now broadly used in transmissions of vehicles such as automobiles. An electric oil pump apparatus compensates for a drop in hydraulic pressure in a transmission caused by stopping idling of a vehicle.
The electric oil pump apparatus includes an electric pump unit formed by unitizing (integrating) an electric motor and an internal gear pump, which is driven by the electric motor to draw and discharge oil (see Japanese Laid-Open Patent Publication No. 2006-188968). In the electric oil pump apparatus, the internal gear pump and the electric motor rotate about a common rotary shaft. This decreases the number of the components, reduces the size of the electric oil pump apparatus, and lowers the cost for manufacturing the apparatus.
In the electric oil pump apparatus, the hydraulic pressure at the discharge side of the internal gear pump may become higher than discharge pressure of the internal gear pump. In this state, an excessive load acts on the electric motor, which drives the internal gear pump. This may cause a loss of synchronism, or irreversible stopping of the electric motor. To solve this problem, a relief valve may be deployed in the electric pump unit to allow fluid to flow back to the suction side of the gear pump if the hydraulic pressure at the discharge side of the internal gear pump becomes greater than or equal to a predetermined value (see Japanese Laid-Open Patent Publication No. 11-13641). The relief valve has a spool (a valve body) that moves toward the suction side of the internal gear pump depending on the hydraulic pressure at the discharge side of the internal gear pump.
However, if such movement of the spool causes a relatively rapid increase of the opening degree of the relief valve, an excessive amount of oil flows back from the discharge side to the suction side of the internal gear pump. This decreases the volume efficiency of the internal gear pump (actual discharge amount of the internal gear pump/theoretical discharge amount of the internal gear pump). Thus, the flow rate of the oil may not satisfy the level required for the transmission of the automobile.
If the chamfer angle, or the angle between a slanted surface formed around a valve head of the spool and a wall surface defining a valve hole for receiving the relief valve, is reduced, the gradient of the linear relation between the movement amount of the spool and the flow rate of the oil becomes more gradual. This prevents the above-described decrease of the volume efficiency of the internal gear pump. However, the loss of synchronism of the electric motor, which is caused by the excessive hydraulic pressure produced at the discharge side of the internal gear pump, cannot be effectively avoided.
To solve this problem, a fluid discharge portion including a small annular opening and a large opening communicating with the small opening may be formed in the valve hole (see Japanese Laid-Open Patent Publication No. 2005-98507). This allows the oil to pass through the portion communicating the discharge side with the suction side of the internal gear pump at an improved passing characteristics.
However, since the fluid discharge portion has a complicated shape, machining the valve hole to form the fluid discharge portion is troublesome. Also, with reference to FIG. 1 of Japanese Laid-Open Patent Publication No. 2005-98507, for example, the relief valve is provided between a discharge port and a suction port, which are at a position spaced from the internal gear pump and extend in complicated manners. This complicates the structure of the electric oil pump apparatus and increases the number of the components of the electric oil pump apparatus. This technique thus cannot meet a recent requirement in a component of an automobile, which is decreasing the size and the weight of the components.